[jerlam]

I always thought the emphasis on breakfast originated in cultures or times where it would be followed by a long day of manual labor.

This also ignores that most European countries emphasize breakfast even less than Americans, and that the British (home of the English breakfast) have a lower life expectancy than most of their western European peers (but not as low as Americans).

[IAmBroom]

Both might be true, but the saying about "most important meal of the day" was genuinely marketing lies.